1. Field of the Invention
The present invention relates to an optical frequency stabilizer and an optical frequency selector which are adapted for application in a wide field inclusive of multiplex optical communication in an optical frequency region and also optical interchange, optical cross connection and optical operation processing at optical frequencies.
2. Description of the Related Art
With regard to the technology of multiplex optical communication, optical interchange, optical cross connection and optical operation processing in an optical frequency region, discussion is currently in progress on the technique of simultaneously stabilizing a multiplicity of optical frequencies and further on the technique of selectively extracting optical frequencies. In a multiplexing system used in the optical frequency region, a function of wavelength conversion is an extremely important technique. As a device having such a function of wavelength conversion, there is proposed a frequency shifter which performs a process of four-light mixing(four-wave mixing). In a system equipped with a frequency shifter of this type, the phase of information of converted signal light is rendered inverse to information of signal light, so that unique optical frequency stabilization and optical frequency selection are requisite.
In the conventional frequency shifters having a function of wavelength conversion based on four-light mixing, there is known a type equipped with a pumping light source to emit pumping light of a frequency f.sub.p and producing converted signal light of a frequency of 2f.sub.p -f.sub.s from the signal light supplied thereto. Generally this frequency shifter further has a nonlinear optical medium which is supplied with both of the pumping light and the signal light, so as to perform four-light mixing by a high power of the pumping light.
In a system equipped with the frequency shifter mentioned above, when the frequency f.sub.s of the signal light and the frequency f.sub.p of the pumping light are stabilized to, e.g., .+-.100 MHz respectively under temperature control, the frequency stability of the converted signal light is .+-.300 MHz since the frequency thereof is 2f.sub.p -f.sub.s. Therefore, the stability is more enhanced by directly stabilizing the frequency of the converted signal light.
Meanwhile the light output obtained from the frequency shifter after the four-light mixing includes signal light of frequency f.sub.s, pumping light of frequency f.sub.p, wavelength-converted signal light of frequency 2f.sub.p -f.sub.s, and another wavelength-converted light of frequency 2f.sub.s -f.sub.p. And it is necessary to select and extract merely the converted signal light of frequency 2f.sub.p -f.sub.s alone out of such four light components.